"Everybody waltz!"

"Everybody waltz!" – this is the invitation to dance at the annual Vienna Opera Ball. This phrase is now applicable to the TerraSAR X and TanDEM X satellites, because as of yesterday the 'dance floor' is open and close formation flight has begun. Upon successful completion of the final tests, permission to commence the now imminent mission phase was granted by a panel of experts at the conclusion of the Formation Flight Reviews.

On Monday, 11 October 2010 at 14:00 UTC we entered uncharted territory. Around this time, the thrusters of TanDEM-X were fired to reduce the separation between it and TerraSAR-X from 20 kilometres to 500 metres. TerraSAR-X remains in its original, circular orbit, while TanDEM-X, moves on a slightly eccentric orbit in a plane that is rotated by a small angle with respect to that of its partner. The result is a 'dance of the satellites' – not in three-quarter time, but something more leisurely.

If one could fly behind the satellites, one would observe that TanDEM X moves around its twin, TerraSAR-X, once every 95 minutes – the duration of their orbit around the Earth. At the North Pole, TerraSAR X overtakes TanDEM X, as the latter, because of its eccentric orbit, is slightly higher and thus orbiting more slowly. At the South Pole, the situation is reversed; TanDEM X orbits lower and faster, and overtakes TerraSAR X. Of course, a collision must be prevented – 'touching' is not allowed! Viewed from side, the two orbits can be imagined as being like two links in a chain – one link round, the other elliptical – intertwined but not touching one another.

To ensure that this works in practice, it has been tested in numerous studies and simulations – made possible by colleagues at the control centre in Oberpfaffenhofen. Our 'Dance Master' – and a specialist on the flight dynamics team – Ralf Kahle, will report in the next blog post – as soon as his heart rate has returned to normal…

The circular orbit of TerraSAR X (red) and the eccentric orbit of TanDEM X (green) never cross, preventing a collision between the two satellites.